fix(plugins): replace dependency marker files with a real satisfaction check

The .dependencies_installed hash-marker system only tracked "was this exact
requirements.txt hashed before" — not whether the packages it names are
actually present. That made it fragile (a wiped venv, a manually removed
package, or a lost/corrupted marker forces a needless full pip reinstall or,
worse, a false skip) and produced dead weight for the ~10 plugins whose
requirements.txt is comment-only (they still paid a pip subprocess on first
boot before a marker existed).

Replace it with requirements_are_satisfied() in plugin_loader.py, which
checks each real requirement line against importlib.metadata directly, so
install_dependencies() only shells out to pip when something is actually
missing or version-mismatched. Drops the marker file entirely: removed all
marker read/write sites in plugin_loader.py and store_manager.py, the
now-pointless marker-cleanup step in the git-update path, the unused legacy
marker implementation in plugin_manager.py, and the already-stale
clear_dependency_markers.sh script.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KEZK1P1Q1fu5pcuVrkrCFZ

requirements.txt

@@ -43,6 +43,9 @@ websocket-client>=1.8.0,<2.0.0
 # JSON Schema validation
 jsonschema>=4.20.0,<5.0.0
 
+# Requirement specifier parsing (plugin dependency satisfaction checks)
+packaging>=23.0,<27.0
+
 # Testing dependencies
 pytest>=9.0.3,<10.0.0
 pytest-cov>=4.1.0,<5.0.0